Method for preparing phosphoric acid

ABSTRACT

A method for preparing phosphoric acid from calcium phosphate ores, particularly ores highly contaminated with organic compounds, by decomposing the ore with sulphuric acid, wherein according to the invention the process of decomposition of calcium phosphate ores with sulphuric acid and circulating phosphoric acid is carried out in oxidizing conditions provided by a constant supply of ozone to the reaction slurry, preferably to the zone of commencing decomposition of the phosphate ores.

The present invention relates to a method for preparing phosphoric acidby a wet method, so called extraction phosphoric acid, from calciumphosphate ores, e.g. phosphorites and apatites and sulphuric acid.

In processes for production of extraction phosphoric acid mainly groundmineral phosphorites or phosphorites enhanced by concentration processesare used. These contain generally fluoroapatite, frequently supplementedwith small quantities of mixed apatites and, depending on the origin,various mineral contaminants such as quartz, calcite, dolomite, loamymatter and variously mineralized organic compounds and/or organiccompounds introduced in the course of concentration.

From the chemical point of view natural calcium fluorophosphates arealways accompanied by great number of various salts of magnesium,aluminium, iron and other cations including rare earth and radioactiveelements. Organic contaminants of phosphorites comprise mainly humus andbituminous compounds with a small concentrations of sulphur (II),phosphorus (III) and nitrogen compounds.

In the process for production of extraction phosphoric acid the basicreactions of decomposition of apatite with sulphuric acid areaccompanied by a number of various simultaneous side reactions of saidcontaminants. These reactions influence greatly the quality ofphosphoric acid and the course and yield of the process for productionof the same.

Organic contaminants of phosphate ores exert a particularly adverseeffect on the process for production of phosphoric acid. Already at theextraction stage they are adsorbed on the surfaces of gypsumcrystallisation, block active sites of crystals and limit capabilitiesof their growth. In consequence of this suspensions hard to filterappear having sticky structure of a filtered precipitate and highcontent of residual phosphoric acid hard to wash out.

The substantial part of organic contaminants is dissolved in resultingphosphoric acid giving it a typical dark-brown colour and enlarging itsviscosity and surface tension. Gaseous reaction products, generallycarbon dioxide, under this conditions bring about an abundant and stablefoam which lowers the rate of extraction and mass yield of the reactor.

Some known and most frequently used mechanical and chemical methods forquenching foam are not always useful and frequently ameliorate thearduousness of the occurrence to a small degree only.

At high temperatures of reaction with phosphoric acid and sulphuric acidthe organic compounds contained in some phosphorites liberate to theflue gases vapours of both saturated and unsaturated fatty acids,various organic amines, sulphides and phosphines distinguishing withtheir strong, repulsive odour. Having been vented with flue gases to theatmosphere, the compounds can be detected even in trace amounts in thedistance of tens of kilometers from the phosphoric acid productionfacility. Attempts to deodorise the vented gases by known methods, e.g.thermal and catalytic oxidation, adsorption on surface active materials,or destruction in bio-reactors failed to achieve success, though themethods are successfully utilised in another branches of industry, e.g.heavy chemical synthesis, tobacco industry, food industry etc.

According to the current state of knowledge the only known efficientmethod of making phosphorites with high content of organic contaminantssuitable for the process for production of extraction phosphoric acid isto precalcine phosphorite in oxidizing conditions at temperatures ofalkaline earth carbonates decomposition. However, the method isenergy-consuming and applied in special cases, e.g. to produce specialextraction phosphoric acid completely devoid of organic contaminants forfurther purification by solvent extraction. Most frequently suchphosphorites are not processed, though they are cheap and readilyavailable, and raw materials more expensive, but less troublesome inprocessing are used.

Therefore, the object of the present invention is to make independentthe process for preparing phosphoric acid by a “wet” method independenceof the type and kind of calcium phosphate ore, and particularly of thetype and amount of organic contaminants contained therein, ateliminating the emission of offensive odours, at the distinct reductionof reaction pulp foaming and at improving the filtrating properties ofresulting phosphogypsum.

This object is achieved through a method for preparing phosphoric acidfrom calcium phosphate ores, especially the ores highly contaminatedwith organic compounds, by decomposing the ore with sulphuric acid,wherein according to the invention the process of decomposition of thecalcium phosphate ores with sulphuric acid and circulating phosphoricacid is carried out in oxidizing conditions by a constant supply ofozone to the reaction slurry, preferably to the zone of commencingdecomposition of the phosphate ores.

The sole FIGURE of the Drawings represents a flow diagram of the processaccording to the invention.

The experiments have shown that organic compounds introduced withphosphorites to the extraction process change their physicochemicalproperties under the influence of strong oxidizers, such as peroxidecompounds, or particularly fast and efficiently under the influence ofgaseous ozone, introduced to the reaction. The oxidation products arealmost completely insoluble in phosphoric acid, thus explaining thedisappearance of adverse effects of organic contaminants. In particularthe viscosity and surface tension are significantly reduced givingdirectly rise to over 60% vanishing of foaming of phosphorites reactingwith acid, and vent gases emerging from quickly vanishing foam arecompletely devoid of the original repulsive odour. At the same time thepresence of ozone causes calcium sulphate dihydrate to precipitate asuniform and well shaped crystals. The slurries thus formed are easy tofiltrate, and resulting filter cakes are easily washed off of the acid.The filtrate acid contains emulsified insoluble organic compounds whichcoagulate readily and sediment with the secondary precipitate. Thesettled acid loses its typical brownish colour getting clear pale greenor olive-green tinge.

One of the advantage of the invention is the fact that it is reliableand effective for various calcium phosphate ores despite of theirorigin, type and kind, and particularly despite of the type andconcentration of organic contaminants.

The additional advantage is the simplicity of the inventive method whichenables using it without any essential modifications of techniques forpreparing extraction phosphoric acid and any structural changes in theapparatus on the production plant except for adding an ozone generatorand the means for feeding ozone to the reactor.

The method is illustrated below by the way of example.

EXAMPLE

The Example illustrates the course of the process in a singlemulti-section model reactor provided with a central two-level bladeagitator for test research of usefulness of calcium phosphate ores forpreparing extraction phosphoric acid according to a scheme depicted onthe enclosed Figure.

To the central chamber 525 kg/h of circulating slurry at 80°C.containing 34% of crystalline CaSO₄x2 H₂O in phosphoric acid at aconcentration of 30% P₂O₅ and 10 kg/h of phosphorite containing 29.5%P₂O_(5,) 32% Ca, 2.8% F, 0.54% organic compounds expressed as elementalcarbon is introduced with continuous stirring, and 0.6 g/h ozone as a16% (w/w) mixture with pure oxygen or a 4% (w/w) mixture with air isconstantly charged. Ozone is introduced just below the level of upperblades of the agitator through a diffuser to diffuse the gas in thehighest degree throughout the reaction slurry. The temperature in thereactor is kept at 82±2° C. The slurry is then supplemented in thecalcium sulphate crystallization zone with 8.8 kg/h sulphuric acid at aconcentration of 95% diluted previously to 56% with a mixed solution ofa phosphoric acid prefiltrate and gypsum washings at a rate of 6.1 kg/h.

After averaging the chemical composition of the slurry in thehomogenization zone 90% of the mass is directed to an adiabaticevaporator to concentrate it and subsequently recirculate to the zone ofthe initial ore decomposition. The rest of the slurry is directed tostages of maturing and subsequent filtration.

The process produces about 9.4 kg/h of extraction sphoric acid at aconcentration of 30% P₂O₅, which after maturing becomes a clear solutionof pale green or olive-green colour containing about 0.03% of organicmatter expressed as carbon. The filtered phosphogypsum at a rate of 29.6kg/h comprising 13.5% P₂O₅ is washed with 21.6 kg/h yielding 22.7 kg ofwashings containing 17% P₂O₅ and mixture 7 kg/h of washed phosphogypsumat a moisture level of 25% containing at average total 0.8 P₂O₅.

What is claimed is:
 1. A method for preparing phosphoric acid fromcalcium phosphate ores contaminated with organic compounds, comprisingmixing said calcium phosphate ores with a slurry comprising phosphoricacid and calcium sulfate in a reactor, treating the calcium phosphateores-containing slurry with ozone and with sulphuric acid, wherein theozone causes calcium sulphate dihydrate to precipitate out, recoveringphosphoric acid from the slurry and recirculating a portion of thetreated slurry to said reactor.
 2. A method for preparing phosphoricacid from calcium phosphate ores contaminated with organic compounds,comprising: introducing said calcium phosphate ores to a slurry andozone in a reactor comprising a central chamber, wherein the slurrycomprises phosphoric acid; mixing the calcium phosphate, slurry andozone to create a reaction slurry; moving the reaction slurry from thecentral chamber to a crystallization zone carried out in oxidizingconditions provided by a constant supply of ozone, wherein the ozonecauses calcium sulphate dihydrate to precipitate out; recirculating partof the reaction slurry to the central chamber to be used as the slurryto introduce other calcium phosphate ores; filtering the reaction slurryremaining in the crystallization zone to obtain the phosphoric acid.